Retained mode scene graphs typically allow lights to be specified as part of a lighting model to be rendered by a processing device. The lighting model may be transformed and composed into a scene in world space, which is a frame of reference of an entire three dimensional (3D) scene, before the 3D scene may be lit.
Two mathematical lighting models, which traditionally do not behave realistically when transformed, are point light models and spot light models. When these light models are used with models of 3D geometry to represent real-world objects, deficiencies with respect to how these lighting models behave when transforms are applied become very apparent.
For example, assume one has a 3D model of a desk lamp sitting on a table. The lamp includes a point light, which will light an area of the table within a fixed radius of the point light, as well as 3D geometry which represents the physical shape and size of the desk lamp. If the desk lamp model, which includes the light, is transformed such that the model is scaled to be half of its original size, one would expect the lit area of the table beneath the lamp to become proportionately smaller to correspond with the lamp's new size. However, in mathematical lighting models, point lights and spot lights are mathematical approximations of a physical light with a theoretical zero area emitter. Therefore, decreasing a size of a point light or a spot light will not affect a radius of a lit area, since the properties on the light model that affect the brightness and attenuation of the light are not transformed. In fact, in the example of the desk lamp having the point light, the lit area beneath the lamp will actually increase when the size of the lamp decreases because, as the size of the lamp decreases, the point light of the lamp moves closer to a surface of the table.